Segregation of RNA and separate packaging of DNA and RNA in apoptotic bodies during apoptosis

Apoptosis is characterized by a complex and remarkably ordered choreography of events consisting of the preparatory and execution steps that all culminate in disposal of the cell remnants. The disposal occurs in a manner that is the least destructive to the tissue: the remains of nuclear chromatin and cytoplasm are packaged in apoptotic bodies which are then phagocytized by neighboring live cells without invoking inflammatory or autoimmune response. In the present study we describe that in the course of apoptosis cellular RNA becomes sequestered and packaged into granules and then into apoptotic bodies, separately from DNA. This separation, which appears to be initiated by the nucleolar segregation, was observed in HL-60 cells that were undergoing spontaneous apoptosis in cultures or were treated with the DNA-damaging drug, DNA topoisomerase I inhibitor camptothecin (CPT), or with the cell death ligand, tumor necrosis factor-alpha. RNA separation was also observed in apoptotic MCF-7 cells following treatment with CPT. RNA and DNA in apoptotic cells were identified histochemically, by their differential stainability with pyronin Y and Hoechst 33342 fluorochromes, respectively, and immunocytochemically, by labeling the RNA with BrU for various periods of time and detection of the incorporated precursor with fluoresceinated anti-BrU mAb; DNA was counterstained with 7-aminoactinomycin D. Over 90% of apoptotic bodies that contained RNA had no detectable DNA and vice versa, the apoptotic bodies containing DNA had no detectable RNA. Packaging RNA and DNA into separate apoptotic bodies suggests that the phagosomes of the cells that ingest these particles are specialized: some of them are responsible for DNA degradation, others for degradation of RNA. Such specialization may facilitate heterophagic degradation of nucleic acids during apoptosis.